Theoretical and experimental techniques are used to develop a bonding model for the interaction of metalloporphyrins with small molecules (CO, NO, O2). This model is utilized in the recognition of behavior unique to hemeproteins. Heme model complexes are designed and synthesized to emulate specific structural or electronic features of hemeproteins as one method of revealing factors essential to protein function. A myoglobin model heme complex has been prepared by the condensation of histamine with chloro hemin. Subsequent reduction yields a high spin five coordinate heme complex in which only one of the two imidazole is bound intramolecularly by Fe(II). Carbon monoxide and nitric oxide derivatives of this heme complex, as well as related model systems, are being fully characterized by chemical and physical methods including X-ray crystal structures. An oxygen complex for this model system may exist in the solid, but oxidizes in solution to Fe(II). Synthetic studies are directed towards developing a heme molecule capable of reversibly binding molecular oxygen.